Active or self-biasing micro-bolometer infrared detector

ABSTRACT

The detector includes a thin-film resistive component ( 3 ), at least two first electrical contacts ( 6, 7 ) electrically connected to the resistive component ( 3 ) that provide for biasing and signal readout, at least one second electrical contact ( 1 ) electrically connected to the resistive component ( 3 ) that provides bias control, an integral infra-red absorption means ( 4, 5 ) and thermal isolation means ( 10, 11 ). The detector may further include a readout integrated microcircuit (RIOC).

BACKGROUND OF THE INVENTION

The present invention relates to a resistance micro-bolometer thermalinfrared detector. Unlike a conventional resistance bolometer that is anelectronically passive device, the present invention is for a detectorthat can be operated in various electronically active modes, hence theterm active bolometer.

The concept of a thin film resistance bolometer has been known for manyyears, and a description can be found in various publications. See, forexample, U.S. Pat. Nos. 4,574,263, 5,369,280, and 5,300,915, for adescription of micro-bolometer operation and recent technologies.

Conventional resistance bolometers are essentially passive devices, inthat they function as a simple resistor component in an electroniccircuit. In operation as an infrared detector the electrical resistanceis caused to change by an incremental amount following radiationexposure as a consequence of the temperature coefficient of resistanceof the detector material, and this resistance change is detected as achange in electrical current flowing through the detector. The operationand performance of the detector is determined by the external appliedbias, signal conditioning, and signal processing circuits.

In the case of a focal plane array of such detectors, small changes inmaterial and dimensional parameters results in ‘fixed pattern’ noise,which must be corrected before the array can be usefully employed. Thefunctionality and detective performance is limited by parameters such asresistivity and temperature coefficient of resistance, which areproperties of the specific heat sensitive material employed in thedetector.

It is an object of the present invention to provide for a bolometer thatovercomes at least some of these problems or provides the public with auseful alternative.

SUMMARY OF THE INVENTION

Therefore in one form of the invention there is proposed an infraredradiation detector including:

-   -   a thin-film resistive component;    -   at least two first electrical contacts electrically connected to        the resistive component so as to provide for biasing and signal        readout;    -   at least one second electrical contact electrically connected to        the resistive component so as to provide bias control;    -   an integral infrared absorption means; and    -   a thermal isolation means.

Advantageously at least one second electrical contact is adapted toadjust the activation energy for electrical conduction in order toenhance the temperature coefficient of resistance of said resistivecomponent.

Preferably the detector functions in a self-amplification mode for thepurpose of signal conditioning.

Preferably the detector is located on a plane above or coplanar with asilicon wafer substrate.

Preferably the detector further includes a readout integratedmicrocircuit (ROIC).

Preferably the thin-film resistive component is formed in crystalline oramorphous silicon.

Preferably the thin-film resistive component is formed in alloys ofsilicon with material selected from the group consisting of hydrogen,nitrogen, germanium, boron, phosphorous, carbon, antimony, tin.

Preferably the resistive component includes a dopant said dopantadjusting the electrical resistance of said resistive component.

Preferably the dopant is formed from phosphorus or arsenic.

Preferably the resistive component is a single film.

Preferably the resistive component is a composite film constructed oftwo or more layers each layer having different electrical conductivity.A composite layer is typically constructed of two or more layers havingdifferent conductivity. The conductivity may be determined by varyingthe dopant or alloy concentration within the layers

Preferably the conductivity of said layers is controlled by varying theamount of dopant or alloy concentration within the layers.

Preferably the integral infrared absorption means includes a backreflector, said resistive component and a top metal layer arranged toform an integral optical interference filter.

Preferably the thermal isolation means includes at least two legsraising said detector above the ground plane.

Preferably the said legs are of a generally small cross-sectional area.

Preferably the bias control modulates or adjusts the level of biascurrent operating in the detector.

Preferably the bias control switches the bias current in accordance withthe signal readout requirements.

Preferably the detector element is adaptable for electronicamplification of the detected signal.

In a further form of the invention there is proposed a two-dimensionalarray of infrared detectors each detector including;

-   -   a thin-film resistive component;    -   at least two first electrical contacts electrically connected to        the resistive component so as to provide for biasing and signal        readout;    -   at least one second electrical contact electrically connected to        the resistive component so as to provide bias control;    -   an integral infrared absorption means; and    -   a thermal isolation means.

Preferably the bias control serves the function of removing fixedpattern noise from said two-dimensional array.

In preference an electronic circuit, which operates the bias control, isan external circuit or formed on a ROIC.

Preferably each detector further including a memory component thatstores bias values optionally as an external component or formed on theROIC.

In preference said detectors are adapted to function in a conventionalpassive mode without the bias control.

In preference the thermal isolation includes two or more legs ofgenerally small cross-sectional area, which extend to pillars elevatedabove the substrate so that thermal isolation is achieved by plasmaetching of a polyimide sacrificial layer.

In a yet further form of the invention there is proposed a method ofproducing an infrared radiation detector substantially as hereindescribed.

Thus the present invention provides for an active bolometer thatgenerally includes at least one control electrode in addition to the twobias electrodes that pass bias current. The device generally functionsin a similar manner to a thin film transistor, in that the two biaselectrodes function as ‘source’ and ‘drain’, and the control electrodeas a ‘gate’, thereby enabling control of bias current and deviceperformance. The ability to operate a bolometer in this manner offersconsiderable advantages in terms of enhanced functionality, versatilityand detective performance.

An improved bolometer is therefore provided by virtue of one or more ofthe following;

-   -   a. controlled switching or modulation of the bias voltage;    -   b. in conjunction with supporting electronics and on-chip        temperature sensors, automatic correction to the bias current to        eliminate fixed pattern noise;    -   c. in conjunction with supporting electronics and on-chip        temperature sensors, provision of automatic corrections to the        bias current to compensate for bias temperature variations;    -   d. the ability to tune the detector to adjust the activation        energy for electrical conduction and hence the temperature        coefficient of resistance; and    -   e. capability of self-amplification of the detected IR signals.

Other objects and advantages will become apparent when taken intoconsideration with the following drawings and specifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several implementations of theinvention and, together with the description, serve to explain theadvantages and principles of the invention. In the drawings,

FIG. 1 is a cross-sectional schematic of an active bolometer accordingto a first embodiment of the present invention and illustrating thevarious layers forming the detector;

FIG. 2 is a plan view illustrating the thermal isolation of saiddetector; and

FIG. 3 is a cross-sectional view of an alternate embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description of the invention refers to theaccompanying drawings. Although the description includes exemplaryembodiments, other embodiments are possible, and changes may be made tothe embodiments described without departing form the spirit and scope ofthe invention. Instead, the scope of the invention is defined by summaryof the invention and appended claims, if any. Wherever possible, thesame reference numbers will be used throughout the drawings and thefollowing description to refer to the same and like parts.

FIG. 1 illustrates a bolometer including a thin film metal controlelectrode 1, which has the dual function of acting as an electricalcontact and optical reflector, bottom insulator layer 2, semiconductorlayer 3, top insulator 4, and top metal film 5. Electrical contact tothe semiconductor layer 3 is made for the purpose of bias and signalreadout by contacts 6 and 7. Heavily doped semiconductor layers 8 and 9may advantageously be placed between the semiconductor layer 3 andcontacts 6 and 7 to obtain low-contact resistance. The detectorstructure may be generally supported on pillars 10 and 11 above a singlecrystal silicon wafer substrate 12. The pillars may be advantageouslysolid metal structures prepared by such processes as copper or aluminiumdamascene technology, or formed by thin film processes such as imagereversal lithography and lift-off.

Generally, it is preferred that the silicon wafer will have formed inits surface a microelectronic circuit for signal conditioning andreadout.

FIG. 2 illustrates one possible arrangement for thermal isolation. Inthis simplified plan view, the metal film 5 is shown on top of thesemiconductor layer 3. The bias contact electrodes 6 and 7 makeelectrical contact to each end and on top of the semiconductor layer andextend to legs 13 and 14. The legs have the dual purpose of electricalinterconnection and thermal isolation, and are supported at the ends bypillars 10 and 11. The control electrode 1 extends to legs 15 and 16,which function in similar manner as legs 13 and 14. It will beunderstood that legs 15 and 16 extend from the bottom of thesemiconductor layer. Because of the inherent strength and mechanicalstability of the four-leg structure, each leg can be formed with smallcross-sectional area to attain a low thermal conductance. However, otherconstructions may equally well be applicable. For example, in a furtherconfiguration four pillars of low thermal conductance may be positionedat the periphery of the detector structure.

The detector will generally be formed on top of a ‘sacrificial layer’such as a polyimide layer (not shown), which is coated on the siliconwafer 12. Vias are opened in the polyimide layer by conventionallithography and reactive ion etching (RIE), and pillars 10 and 11 extendthrough the vias to the underlying silicon wafer.

The following description provides information as to one possible orderof manufacture of the detector components.

Metal film 1 is first deposited using conventional lithography andpatterning by either lift-off or sputter etching. The preferred metalsare nickel-chrome alloy or titanium. The thickness of this layer must besufficient to act as an infrared reflector in the waveband 8 to 13 μm,and this requirement is met if the sheet resistance of the film exceeds10 ohms per square. Dielectric layer 2 is in preference silicon nitride,but may be another suitable dielectric material such as silicon carbide,and is patterned using RIE. The semiconductor material forming layer 3will be an amorphous silicon or silicon alloy, including one of:

-   -   a. phosphorous or boron gas-phase doped amorphous silicon        hydrogen alloy, a-Si:H, prepared by plasma enhanced chemical        vapour deposition (PECVD) or physical deposition (sputter or        thermal evaporation);    -   b. alloys of silicon with germanium; and    -   c. an alloy of silicon with nitrogen, boron, phosphorous,        carbon, antimony or tin, prepared by co-deposition of silicon        and the alloying material by physical vapour deposition

The ability to vary the electrical resistivity and activation energy ofsilicon by gas phase doping or alloying with germanium is well known.The use of silicon alloyed with nitrogen, boron, phosphorous, carbon,antimony or tin for resistance bolometers provides a novel alternativeof which only nitrogen has been previously been known in the presentapplication. Varying the alloying metal content significantly alters theelectrical properties of the material to meet specific performancerequirements. These alloys may or may not have hydrogen within thematerial. One objective of this material embodiment is to reduce theamount of hydrogen or eliminate it completely for the purpose ofachieving low values of excess noise.

Microcrystalline silicon and polycrystalline silicon are cited asoptional to amorphous silicon.

Following deposition and patterning of the silicon layer, layers 8 and 9may be deposited and patterned. A low contact resistance is achieved byconventional methods such as gas-phase doping or ion implantation, or byusing low resistivity alloys formed of the materials cited above forlayer 3. Layer 4 is in preference silicon nitride, but other suitabledielectric such as silicon carbide may be employed. Metal layer 5 is inpreference nickel-chrome alloy or titanium, but other metals includingplatinum may be suitable alternatives. This layer is semi-transmittingto infrared radiation in the waveband 8 to 13 μm, and has a nominalsheet resistance of 377 ohm per square.

The detector ‘stack’ comprising layers 1,2,3,4 and 5 comprises anintegral optical interference filter (also known as an optical cavity)for absorption of infrared radiation in the desired waveband. This is adesirable, although not an essential feature of the invention. It may benoted that the top metal 5 is optional, in particular if layer 4 has theappropriate absorption index to meet the requirement for infraredabsorption.

The final operation is to remove the sacrificial layer by plasmaetching, causing the detector to be thermally isolated from theunderlying substrate.

In an alternative arrangement, shown in FIG. 3, layers 2, 3, 4 and 5 maybe deposited in sequence and patterned using the same photo mask, themetal film 5 being sputter etched and the other films patterned usingRIE.

It is to be understood that the technology described herein can beapplied to either single detector elements or two-dimensional arrays ofdetectors. The latter may be in the form of sparse or close-packedarrays. A sparse array enables some versatility in the means for thermalisolation. For example, the folded legs shown in FIG. 3 must be used forclose-packed arrays, but for sparse arrays the legs can extend outwardsto any convenient location. A large number of detectors or detectorarrays can be fabricated by step-and-repeat lithographic methods, andthen diced into separate chips for mounting, bonding and packaging.

In other embodiments more than one control electrode may be employed toenable dual control functions in the same detector element. It will beunderstood by those skilled in the art that a detector prepared by themethods described will also function in passive mode, so that thisspecification is intended to include new methods for fabrication ofmicro-bolometers in general.

Advantageously, the detector or detector array will be integrated on thesame silicon wafer as the associated ROIC, however, one or more of thesignal conditioning and readout circuits may be located off-chip.Furthermore signal-processing circuits may also be located on or off thedetector chip.

The detector may be operated with either a steady state (DC) ormodulated bias current applied to the bias electrodes. A voltage appliedto the control electrodes will alter the conductivity of thesemiconductor, by virtue of the well-known theory of thin film fieldeffect transistors, thereby changing the bias current and signalvoltage. This characteristic is particularly advantageous for an arrayof detectors where, with the aid of associated signal processing, thebias may be adjusted on a detector-to-detector basis to remove fixedpattern noise arising from small variations in the detective performanceof individual detectors at a given operational temperature. Furthermore,it is possible to adjust for larger signals that occur when the detectortemperature changes, thus removing the requirement for temperaturestabilization.

The ability to control bias current also suggests that applying amodulated control voltage could modulate the bias current. Thisalternative embodiment might be advantageous in that the signal wouldalso be modulated and detectable by techniques such as phase sensitivedetection.

In some applications it may be advantageous to change the activationenergy for electrical conduction of the semiconductor layer. This may beuseful to adjust for variations that occur in semiconductor preparation,where the properties are difficult to control, or where the appliedcontrol voltage shifts the properties into a region of higher activationenergy, hence sensitivity.

Those skilled in the art will appreciate that the present inventionprovides for significant improvements over existing technology,including, but not limited, to the following:

-   -   a. the ability to tune the electrical resistance of the detector        to match optimum input impedance of the associated ROIC;    -   b. the activation energy may be tuned to give a desired        temperature coefficient of resistance; thus when applied to a        focal plane detector array the uniformity of response across the        array can be enhanced;    -   c. by integrating the detector with an appropriately designed        ROIC it will be possible to utilise the ability to control the        resistance and activation energy in order to self-bias the        circuit, taking the form of a feedback circuit to compensate for        changes in resistance and activation energy due to ambient        temperature drift; and    -   d. ability of the detector to act as a switch.

Further advantages and improvements may very well be made to the presentinvention without deviating from its scope. Although the invention hasbeen shown and described in what is conceived to be the most practicaland preferred embodiment, it is recognized that departures may be madetherefrom within the scope and spirit of the invention, which is not tobe limited to the details disclosed herein but is to be accorded thefull scope of the claims so as to embrace any and all equivalent devicesand apparatus.

1. An infrared bolometer radiation detector which functions as athin-film field-effect transistor comprising: a thin-film semiconductorcomponent; at least two first electrical contacts connected to thesemiconductor component to act as source and drain electrodes; at leastone second electrical contact to function as a gate electrode; a firstthin-film insulator layer between the gate electrode and semiconductorlayer; a second thin-film insulator layer deopsited on the semiconductorlayer on the opposite surface to that of the first insulator layer; atop metal film deposited onto the second insulator layer; an integralabsorption means; and a thermal isolation means.
 2. An infraredradiation detector as in claim 1, wherein said at least one secondelectrical contact is adapted to adjust the activation energy forelectrical conduction in order to enhance the temperature coefficient ofresistance of said semiconductor component.
 3. An infrared radiationdetector as in claim 1, wherein said detector functions in aself-amplification mode for the purpose of signal conditioning.
 4. Aninfrared radiation detector as in claim 1, wherein said detector islocated on a plane above or coplanar with a silicon wafer substrate. 5.An infrared radiation detector as in claim 1, wherein said detectorfurther includes a readout integrated microcircuit (ROIC).
 6. Aninfrared radiation detector as in claim 1, wherein the thin-filmsemiconductor component is formed in crystalline or amorphous silicon.7. An infrared radiation detector as in claim 1, wherein the thin-filmsemiconductor component is formed in alloys of silicon as distinct fromdopants, with material selected from the group consisting of hydrogen,nitrogen, germanium, boron, phosphorous, carbon, antimony and tin.
 8. Aninfrared radiation detector as in claim 1, wherein said semiconductorcomponent includes a dopant, said dopant adjusting the electricalresistance of said semiconductor component.
 9. An infrared radiationdetector as in claim 8, wherein said dopant is formed from phosphorus orarsenic.
 10. An infrared radiation detector as in claim 1, wherein saidsemiconductor component is a single film.
 11. An infrared radiationdetector as in claim 1, wherein said semiconductor component is acomposite film constructed of two or more layers, each layer havingdifferent electrical conductivity and formed during the same depositionprocess.
 12. An infrared radiation detector as in claim 11, wherein theconductivity of said layers is controlled by varying the amount ofdopant or alloy concentration within the layers.
 13. An infraredradiation detector as in claim 1, wherein said integral absorption meansincludes a back reflector formed by the gate electrode, saidsemiconductor component and the top metal film arranged to form anintegral optical interference filter.
 14. An infrared radiation detectoras in claim 1, wherein the thermal isolation means includes at least twolegs raising said detector above the ground plane.
 15. An infraredradiation detector as in claim 14 wherein said legs are of a generallysmall cross-sectional area.
 16. An infrared radiation detector as inclaim 1, wherein a voltage applied to the gate electrode modulates oradjusts the level of bias current operating in the detector.
 17. Aninfrared radiation detector as in claim 1, wherein a voltage applied tothe gate electrode switches the bias current in accordance with thesignal readout requirements.
 18. An infrared radiation detector as inclaim 1, wherein the detector is adaptable for electronic amplificationof the detected signal.
 19. A two-dimensional array of infrareddetectors wherein each detector comprises: a thin-film semiconductorcomponent; at least two first electrical contacts connected to thesemiconductor component to act as source and drain electrodes; at leastone second electrical contact to function as a gate electrode; a firstthin-film insulator layer between the gate electrode and semiconductorlayer; a second thin-film insulator layer deposited on the semiconductorlayer on the opposite surface to that of the first insulator layer; atop metal film deposited onto the second insulator layer; an integralabsorption means; and a thermal isolation means.
 20. A two-dimensionalarray of infrared detectors as in claim 19 wherein a voltage applied tothe gate electrode serves the function of removing fixed pattern noisefrom said two-dimensional array.
 21. A two-dimensional array of infrareddetectors as in claim 19 wherein an electronic circuit which controlsthe voltage applied to the gate electrode is an external circuit or isformed on a readout integrated microcircuit (ROIC).
 22. Atwo-dimensional array of infrared detectors as in claim 19, wherein eachdetector further includes a memory component that stores bias valuesoptionally as an external component or formed on the readout integratedmicrocircuit (ROIC).
 23. A two-dimensional array of infrared detectorsas in claim 19, wherein said detectors are adapted to function in aconventional passive mode without a voltage being applied to the gateelectrode.